In general, when light strikes a surface, some of it may be reflected, some absorbed, some scattered, and the rest transmitted. Reflection can be diffuse, such as light reflecting off a rough surface such as a white wall, in all directions, or specular, as in light reflecting off a mirror at a definite angle. An opaque substance transmits almost no light, and therefore reflects, scatters, or absorbs all of it. Both mirrors and carbon black are opaque. Opacity depends on the frequency of the light being considered. “Blackout” or light blocking materials typically refer to coated layers in articles that are substantially impermeable to light such as visible or UV radiation. Thus, when a blackout material such as a blackout curtain is hung over a window, it generally blocks substantially all external light from entering the room through that window. Blackout materials are suitable as curtains and shades for domestic use, for institutional use in hospitals and nursing homes, as well as for use in commercial establishments such as hotels, movie theaters, and aircraft windows where the option of excluding light can be desirable.
Light blocking articles such as the blackout curtains can be comprised of a fabric (porous) substrate coated with more than one layer of a foamed latex composition. There is a desire for these curtains, in addition to blocking transmitted light, to have a light color (hue) facing the environment when an activity needs illumination so as to minimize the amount of artificial lighting needed to perform the activity. An example is when the function of the blackout material is to separate two areas of activity where one or both areas can be artificially lit at the same time. However, more often than not, the function of a blackout curtain is to prevent sunlight from entering a room through a building window. It can also be desirable for the color (hue) of the back side to match the external decor of the building.
Light colored blackout curtains can be made by coating a fabric with light colored foams containing light scattering pigments such as titanium dioxide or clays. However, very thick foam coatings will be needed to create blackout curtains through which the sun is not visible in a darkened room using only these pigments. One method that is used to reduce the weight of such blackout materials is to sandwich a light-absorbing, foamed black or grey pigment, such as carbon black layer between two light scattering, white pigment-containing layers.
When an electromagnetic radiation blocking coating has, as it often does, a strongly light absorbing material containing black pigments such as carbon black, between two reflective layers, it has at least two distinct problems. First, such materials require three separate coating operations that reduce manufacturing productivity and increase unit costs. Secondly, carbon black in the light absorbing middle layer can become “fugitive” (or non-enclosed) from some puncture or tear occurring during sewing or laundering, and soil other layers such as the reflective layers, which is highly objectionable. Additionally, the stitches generated in the materials during sewing can cause the fugitive carbon from the light absorbing layer to spread over a larger area thereby increasing the area of objectionable shading of the light colored surface.
U.S. Pat. No. 7,754,409 (Nair et al.), U.S. Pat. No. 7,887,984 (Nair et al.), U.S. Pat. No. 8,252,414 (Putnam et al.), and U.S. Pat. No. 8,329,783 (Nair et al.) describe porous polymer particles that are made by a multiple emulsion process, wherein the multiple emulsion process provides formation of individual porous particles comprising a continuous polymer phase and multiple discrete internal pores, and such individual porous particles are dispersed in an external aqueous phase. The described Evaporative Limited Coalescence (ELC) process is used to control the particle size and distribution while a hydrocolloid is incorporated to stabilize the inner emulsion of the multiple emulsion that provides the template for generating the pores in the porous particles.
U.S. Patent Application Publication 2015/0234098 (Lofftus et al.) describes improved articles that are designed with an opacifying layer that is capable of blocking predetermined electromagnetic radiation. The opacifying layer is disposed on a substrate that can be composed of any suitable material and a porous or non-porous underlying layer can be incorporated between the substrate and the opacifying layer. While these articles have numerous advantages and represent an important advance in the art, there is a need for further improvement in providing opacifying articles that are lighter in weight; and that have improved flexibility, good “hand”, while maintaining light coloration of the surfaces facing an observer without losing reflectivity, and light-absorptive properties; launderability; and minimizing dark opacifying agents getting out into the environment upon stitching and handling.
An improvement in this art is provided by the foamed aqueous compositions described and claimed in recently allowed U.S. Ser. No. 15/144,875 (noted above) in which very small amounts of opacifying colorants can be incorporated into porous particles, and the resulting composition has a foam density of at least 0.1 g/cm3.
While the noted foamed compositions and foamed, opacifying elements described in the previous commonly assigned patent applications provide an advance in the art, there is continued need for improvements. There is a need for superior opacifying compositions and articles containing porous particles that provide improved light scattering and opacity to a dry opacifying layer by maintaining the porosity and pores in the porous particles during high temperature drying and long residence times in the dryers, after applying the foamed aqueous composition on porous substrates. There is also a need to have higher porosity derived from a large number of smaller pores that can result in better light scattering to provide greater luminosity in foamed, opacifying elements.